Following the upgrade of my laptop to Ubuntu 8.04, Zoe found that Runescape would no longer run on Firefox.

This thread on Ubuntu Forums suggested that the problem was that icedtea-gcjwebplugin was being used to run Java applets instead of the sun-java6-plugin. I started the Synaptic package manager and completely removed the icedtea-gcjwebplugin package (the sun-java6-plugin was already installed).